First, you figure out the basis. Generally speaking, the house basis would get stepped up to the value of the house as of the husband's date of death. Whether this is a full step up in basis or only half depends on whether their state of residence is a community property state or not.
Then you can take the sales price minus the basis to figure the capital gain. Since she's single and her husband died more than 2 years ago, she would generally only be eligible to exclude $250K of gain.
Anything not excluded would be a long term capital gain.
If the gain is over that $250K exclusion amount, there are additional knobs to turn. Certain selling expenses, most notably realtor fees but there are others, can be subtracted from the gain as well. Home improvements made after the husband's death would also be subtractable. See IRS Pub 523 for details.
In most cases, even if she can exclude the entire gain, she would need to report the sale on her tax return on Form 8949 and Schedule D. Any taxable gain is taxed just as if she had sold a stock or mutual fund at a gain - it stacks on top of her ordinary income and is taxed at capital gains rates and brackets. You can plug in her income and gain here to see how it works:
https://engaging-data.com/tax-brackets It won't be quite right, because she's probably over 65 so her standard deduction will be a bit bigger, but it'll give you the gist of it.